AP Biology 2010-2011
MeasuringEvolution of Populations
Hardy Weinberg
AP Biology
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
Migration Sexual Selection
Small populationNatural Selection
Differential Survival
ChemicalChanges to DNA
AP Biology
Researchers know whether or not a population is evolving by tracking deviations from a baseline of genetic equilibrium.
If you notice, they violate all the mechanisms of microevolution.
Five conditions required for a stable gene pool:1. Mutations do not occur2. Population is infinitely large (genetic drift eliminated)3. No gene flow4. Random mating (no sexual selection)5. All individuals survive and reproduce equally (no NS)
If these 5 things happen, new alleles are not introduced to a population and therefore, the population isn’t evolving.
This doesn’t happen in reality but the rate these change has a direct affect on the rate of evolution.
III. Genetic Equilibrium
AP Biology
Genotype Frequencies Vs. Allele FrequenciesStudying population genetics requires understanding alleles in a gene pool.Each individual has 2 alleles for each trait.This is the genotype, the phenotype is dependent upon this.The gene pool represents all available alleles in a population so genotypic and allelic frequencies always add up to 1.
Genotype Frequency(frequency of EE) + (frequency of Ee) + (frequency of ee) = 1
Allele Frequency: (frequency of E) + (frequency of e) = 1
AP Biology
The Hardy-Weinberg Formula. Rules
Determining Genetic Equilibrium… the BASELINE The Hardy-Weinberg formula can be used to
determine if a population is in genetic equilibrium
p2 + 2pq +q2= 1.0 is essentially: (AA) + (Aa) + (aa) = 1.0
It should be noted that H-W examines only traits that have 2 alleles! We couldn’t use it for our turtle population b/c there
were 6 phenotypes.
The frequency of the dominant allele (A) plus the recessive allele (a) equals 1.0
p + q = 1.0
Homozygous dominant
Homozygous recessive
Heterozygote
AP Biology
The Hardy-Weinberg Principle The Hardy-Weinberg
principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving.
AP BiologyFig. 18-3a, p. 280
Finding out whether a population is evolving. The frequencies of wing-color alleles among all of the individuals in this hypothetical
population of morpho butterflies are not changing; thus, the population is not evolving.
AP BiologyFig. 18-3b, p. 280
490 AA butterflies dark-blue wings
490 AA butterflies dark-blue wings
490 AA butterflies dark-blue wings
420 Aa butterflies medium-blue wings
420 Aa butterflies medium-blue wings
420 Aa butterflies medium-blue wings
90 aa butterflies white wings
90 aa butterflies white wings
90 aa butterflies white wings
Starting Population Next Generation Next Generation
AP Biology
Class Example 1 Let’s see if we can figure out allele frequencies for a population of pigs using H-W.
The goal is to find: q2, p2, p, q, 2pq,
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Example: White (pink) coats are dominant over black. Can you identify who is dominant and recessive. Yes. Can you determine who has what alleles?
Only for the homozygous recessive!
You have to calculate the others….
AP Biology
Hardy-Weinberg Sample Pink is dominant over black. Calculate q2: Count the individuals that are
homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q2.
q2 = 25%
AP Biology
Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele.
q = .5
AP Biology
Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the dominant allele?
p = 1 – qp = 1 - .5
p = .5
AP Biology
Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat:
q = .5p = .52pq = 2(.5)(.5)2pq = .5
This represents the carriers of the recessive alleles
AP Biology
This shows the allele frequency in generation 1.
One would study the allele frequencies over several generations to see if the population is evolving.
AP Biology
What is the Point of H-W? Why we use the H-W equation is to
determine if a population is evolving. By calculating the equilibrium in the
beginning of a population we know the distribution of alleles.
If we examine a population later in time we can see if there is a change.
According to H-W, if there is no change there is no evolution.
If there is change evolution has occurred.
AP Biology
Independent Practice Complete Question #1 on the handout. You may ask questions – share out answers. The goal is to find: q2, p2, p, q, 2pq You start by determining the % of
homozygous recessives in a population (this is q2)
It is also important why you are doing this… it is to determine the genetic equilibrium baseline for the population.
Complete the rest of the worksheet for classwork/homework.
AP Biology
Example Of Strong Selection Pressure Tay Sachs
Primarily in Ashkenazi Jews & Cajuns Recessive disease = aa lysosomal storage disease
lack of one functional digestive enzyme in lysosome
build up undigested fat in brain cells
children die before they are 5 years old
So where do new cases of Tay-Sachs come from?
AP Biology
Example of heterozygote advantage Sickle cell anemia
inherit a mutation in gene coding for one of the subunits in hemoglobin oxygen-carrying blood protein normal allele = Hb
mutant allele = Hs
recessive trait = HsHs
low oxygen levels causes RBC to sickle clogging small blood vessels damage to organs
often lethal
AP Biology
Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
1
2
3
liver
AP Biology
Sickle cell frequency High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs
usually die before reproductive age
Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous… HbHs
Suggests some selective advantage of being heterozygous… HbHs
AP Biology
Heterozygote Advantage In tropical Africa, where malaria is common:
homozygous dominant (normal) reduced survival or reproduction from malaria: HbHb
homozygous recessive reduced survival & reproduction from sickle cell anemia: HsHs
heterozygote carriers survival & reproductive advantage: HbHs
Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele &
distribution of malaria
AP Biology
Concept Check What is the usefulness of the H-W Law?
Homework Complete the H-W Online Tutorial by
your next period. http://www.montereyinstitute.org/
courses/AP%20Biology%20I/course%20files/multimedia/lesson23/lessonp.html?showTopic=2
AP Biology 2010-2011
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